A typical linear equation has the following (point-slope) form
y=mx+b
where
y=result (dependent variable), temperature in this case
x=the value of the variable, time in this case We use t instead of x for the variable name
m=slope, i.e. how much y varies when x increases by a unit (1).
b=initial value, i.e. value of y when x (or t) = 0.
Therefore, in the given case,
initial value = 100 degrees
Question says temperature decreases uniformly from 100 to 25 degrees in 5 seconds, i.e. it DROPPED 75 degrees in 5 seconds, or it was 75/5=15 degrees LESS per second, so m=-15.
So we have just found the equation to describe the temperature with respect to time,
y=mt+b
=(-15)t+100
Said in words, we say temperature C = 100 minus 15t.
